The present invention relates to a refrigerant compressor adapted to operate using a 1,1,1,2-tetrafluoroethane (hereinafter referred to as HFC134a) or a 1,1-difluoroethane (hereinafter referred to as HFC152a) as a refrigerant and a refrigerator oil in which the foregoing refrigerant is soluble.
Generally, a room air conditioner, automobile air conditioner, refrigerator, and so forth use a refrigerant compressor for blowing cold air or hot air. As refrigerant compressors, hermetic type refrigerant compressor, automobile type semi-hermetic refrigerant compressor, and so forth are known.
A typical hermetic type rotary refrigerant compressor as shown in FIG. 1 that is a vertical sectional view will be described below as an example.
Referring to FIG. 1, a drive motor (not shown) is accommodated in a casing 1. A shaft 2 to be rotated by the drive motor (not shown) extends through a cylinder 4 while it is rotatably supported by bearings, and the lowermost end of the shaft 2 is rotatably supported by a subbearing 5.
The shaft 2 includes a crank portion (eccentric portion) in the cylinder 4. A roller 6 fitted between the crank portion and the cylinder 4 conducts planetary movement as the shaft 2 is rotated.
In addition, the refrigerant compressor includes a blade 7 which extends through the cylinder 4. The inner end of the blade 7 comes in contact with the outer periphery of the roller 6 under the effect of the biasing force given by a spring 8, whereby the interior of the cylinder 4 is divided into a suction chamber and a discharge chamber by the blade 7. As the roller 6 conducts planetary movement, the blade 7 moves reciprocably.
As the roller 6 conducts planetary movement, a refrigerant gas is introduced into the refrigerant compressor via a suction port (not shown) and the compressed refrigerant gas is discharged through a discharge port (not shown).
To smoothen slidable movement of slidable portions in the refrigerant compressor, a refrigerator oil 9 is contained in the casing 1. As the shaft 2 is rotated, the refrigerator oil 9 is sucked up by a pump 10 fixedly mounted on the lower end of the shaft 2 to lubricate the slidable portions with the refrigerator oil 9.
An abrasion phenomenon appearing in the refrigerant compressor as constructed in the above-described manner is attributable to two causes associated with the blade 7 and the shaft 3.
The first cause is based on the fact that as the shaft 2 is rotated, the blade 7 reciprocably moves while coming in rubbing contact with the inner wall surface of the cylinder 4 under the effect of the differential pressure arising across the two chambers in the cylinder 4. In other words, the blade 7 slidably moves during its reciprocable movement while coming in local contact with the inner wall of the cylinder 4 within the annular clearance between the cylinder 4 and the roller 6. Because of the local contact, a high intensity of pressure (large load) arises on the slidable surface portion between the blade 7 and the cylinder 4. The reciprocable slidable movement of the blade 7 arises at two stop locations where the slidable speed of the blade 7 is reduced to a zero level. Because of the aforementioned two reasons, the surface of each slidable member is plastically deformed and the lubricant film is broken, causing the slidable parts to readily come in metallic contact with each other. For this reason, the blade 7 and the cylinder 4 are liable to readily be abraded. In addition, since the blade 7 is squeezed against the roller 6 by the spring 8 at its one end, the outer periphery of the roller 6 is liable to readily be abraded too.
The second cause is based on the fact that the shaft 2 is rotated at a high speed in the slightly bent state because it receives the resilient force of the spring 8 and the pressure in the cylinder 4 via the roller 6, causing it to be squeezed against the frame 3 and the bearing 5. At this time, the lubricant film is broken, whereby the surface of the shaft 2 readily comes in metallic contact with the frame 3 and the subbearing 5. Consequently, the outer surface of the shaft 2, the inner surface of the frame 3 and the inner surface of the subbearing 5 are liable to be abraded.
S. C. Kang and K. C. Ludema investigated the mechanisms of "break-in" of lubricant surfaces between a steel cylinder and a flat steel surface (S. C. Kang and K. C. Ludema, Wear, pages 375-384, 108(1986)) However, they did not published a report on Fe.sub.3 O.sub.4 which was formed on a substrate of ferrous metallic material having a surface hardened layer.
A piston ring for an internal combustion engine having a nitrided layer formed on the slidable surface thereof, and moreover, having a layer of Fe.sub.3 O.sub.4 formed on the surface thereof has been hitherto known (refer to an official gazette of Japanese Unexamined Publication Patent (Kokai) NO. 1-48388). However, the prior invention is concerned with an internal combustion engine and nothing is disclosed on the relationship not only between the piston ring and a refrigerant but also between the piston ring and a refrigerator oil.
A dichloro-difluoromethane (hereinafter referred to as CFC12) and a monochloro-difluoromethane (hereinafter referred to as CFC22) have been hitherto mainly employed as a refrigerant for the hermetic type refrigerant compressor as mentioned above. In addition, a naphthene based mineral oil and a paraffin based mineral oil in which CFC12 and CFC22 are soluble have been employed as a refrigerator oil to be contained in the casing of the refrigerant compressor.
In case that CFC 12 is used as a refrigerant, chlorine atoms in CFC12 react with iron atoms in a substrate of metallic material to form a lubricant film composed of an iron chloride. The lubricant film composed of iron chloride has self-lubricability and exhibits excellent abrasion resistance so that it prevents an occurrence of metallic contact between the slidable members when a high intensity of pressure (large load) is exerted on them and a speed of slidable movement of the slidable members is reduced to a level of zero. Thus, the lubricant film of iron chloride effectively functions to prevent abrasion of the slidable members. In addition, since the conventional refrigerant of CFC12 and the conventional refrigerator oil do not have a polarity, they have low moisture absorbability.
Therefore, the iron chloride film formed on the substrate of ferrous metallic material can be present as a stable film without any occurrence of hydrolysis.
A slidable member having such a three-layered structure that a layer of iron nitride is formed on a substrate of ferrous metallic material, a layer of oxynitride is formed on the iron nitride layer and a porous layer of Fe.sub.3 O.sub.4 is formed as an outermost layer has been disclosed (refer to an official gazette of U.S. Pat. No. 4,944,663). This slidable member is intended to prevent a harsh boundary lubricating condition from arising in a refrigerant compressor by retaining a naphthene based refrigerator oil in the porous layer of Fe.sub.3 O.sub.4 having a comparatively heavy thickness. However, the foregoing prior invention does not disclose a refrigerant compressor wherein a refrigerant of HFC134a or HFC152a and a refrigerator oil in which the refrigerant is soluble are employed therefor.
As is well known, in recent years, it has been found that emission of CFC based refrigerant to the environmental atmosphere leads to the destruction of an ozone zone which has a serious effect not only on human beings but also animals and plants. In view of the foregoing circumstances, it has been determined as an international policy that use of CFC12 and similar materials each having a high ozone depletion potential is stepwise reduced and the use of CFC12 and similar materials is strictly inhibited in future.
To cope with the present situation as mentioned above, a variety of development works have been conducted for providing refrigerants such as HFC134a, HFC152a each to be substituted for the refrigerant of CFC12. Each of HFC134a and HFC152a does not have chlorine atoms contained in each molecule, causing its ozone depletion potential to be reduced to a zero level. In addition, since thermal properties of each of HCF134a and HFC152a as a refrigerant are similar to those of CFC12, there is no need of largely changing the design of a compressing mechanism in the refrigerant compressor. Consequently, it is very advantageous to employ HFC134a and HFC152a as an alternative refrigerant to be substituted for CFC12.
In addition to the development of HFC134a and HFC152a, it becomes important to develop a material for the refrigerant compressor suitably employable for the alternative refrigerant as mentioned above. On the other hand, it is necessary to prevent a refrigerator oil from remaining in a refrigerating cycle during running of the refrigerant compressor, and moreover, return the refrigerator oil to a compressing mechanism in the refrigerant compressor without fail so as to properly lubricate and cool the compressing mechanism. Due to the foregoing necessity, when HFC134a or HFC152a is used as a refrigerant, a refrigerator oil to be used for the refrigerant compressor is required that the refrigerant is soluble therein. However, HFC134a and HFC152a are hardly dissolved in a mineral oil that is the conventional naphthane based refrigerator oil. In view of this fact, practical use of a polyether based oil, a polyester based oil and a fluorine based oil in which HFC134a and HFC152a are soluble has been tried.
However, when the HFC based refrigerant such as HFC134a and HFC152a and the refrigerator oil such as a polyether based oil, a polyester based oil or the like in which the HFC based refrigerant is soluble are used for the refrigerant compressor, there arises a problem that abrasion resistance of a ferrous metallic material such as a cast iron, a carbon steel, an alloy steel, a sintered alloy, a stainless steel or the like is increased, resulting in the refrigerant compressor failing to stably operate for a long time.
The following facts are considered as a cause for the foregoing problem.
Firstly, in case that CFC12 is used as a refrigerant, a film of iron chloride formed on a substrate of metallic material has self-lubricability and exhibits excellent abrasion resistance. On the other hand, in case that HFC134a or HFC152a is used as a refrigerant, since no chlorine atom is present in the refrigerant, a lubricant film composed of an iron chloride is not formed on the metallic substrate.
Secondly, a cyclic compound is contained in the naphthane based refrigerator oil, and it has a high ability of forming an oil film. On the other hand, since the refrigerator oil in which HFC134a or HFC152a are soluble is a chain compound containing no cyclic compound, it has a low ability of forming an oil film. For this reason, it is impossible to hold the oil film under a severe condition of slidable movement. In view of this fact, an additive such as an extreme pressure additive is added to the polyether based oil and the polyester based oil.
Generally, the polyether based oil and the polyester based oil have a high moisture absorbability, causing an organic insulating material using in the refrigerant compressor to be readily hydrolyzed. When the metallic substrate has a porous layer of Fe.sub.3 O.sub.4 having a comparatively heavy thickness usable for a combination of the CFC based refrigerant with the naphthane based refrigerator oil, a hydrolyzed product is readily entrapped on the layer composed of Fe.sub.3 O.sub.4, resulting in the lubricating property of the refrigerator oil being degraded. In addition, an additive to be added to the polyether based oil, the polyester based oil or the like is liable to promote hydrolysis of the organic insulating material. Thus, the lubricating property of the refrigerator oil is additionally degraded.